He turned. Peter and the rest were following Christ, as he walked onward. Now Jesus stops, turns, and faces them. Get thee behind me, Satan. Jesus uses nearly the same words in rebuking Peter that he had used to the devil in his temptation (Matthew 4:10); and justly, because the apostle was acting the adversary's part, by opposing the Divine economy, and endeavouring to persuade Jesus that the way he proposed was wholly unnecessary.
The lively stone has became a very Satan in opposing the Divine will; hence the sharpness of the rebuke administered to him. An offence unto me ( σκάνδαλον ἐμοῦ); my stumbling block. Petros, the stone, to maintain the metaphor, is now "a stone of stumbling, and a rock of offence" (1 Peter 2:8).
He stood in the Saviour's way, and impeded his onward progress in the course ordained. He who would turn him aside from Calvary is the enemy of man's salvation, which was to be won there. Thou savourest ( φρονεῖς) not; mindest not (as Romans 8:5); thy taste is not for the Divine plans, but for human considerations; thou art not promoting the great purpose of God, but worldliness and self-pleasing.
"Peter," says St. Chrysostom, "examining the matter by human and earthly reasoning, accounted it disgraceful to him [Christ] and an unmeet thing. Touching him therefore sharply, he saith, 'My Passion is not an unmeet thing, but thou givest this sentence with a carnal mind; whereas if thou hadst hearkened to my sayings in a godly manner, disengaging thyself from thy carnal understanding, thou wouldst know that this of all things most becometh me.
For thou indeed supposest that to suffer is unworthy of me; but I say unto thee, that for me not to suffer is of the devil's mind;' by the contrary statements repressing his alarm" (Oxford transl.).